Gaseous hydrocarbons, particularly those produced in industrial operations, are characterized generally as admixtures of hydrocarbons in varying concentration, inclusive of nonhydrocarbon components. Many include acid gas components which must be removed. Carbon dioxide and other acid gas components such as H.sub.2 S, COS and SO.sub.2 often occur in admixture with hydrocarbons, notably methane, as in natural gas or synthetic natural gas, and must be separated from the hydrocarbon gas prior to its commercial use, e.g., as a fuel. A process of outstanding importance in this regard requires the separation of carbon dioxide and other acid gas components from a mixture of methane and synthesis gas (an admixture of carbon monoxide and hydrogen). The separation of carbon dioxide from such mixtures is quite burdensome particularly since it is often contained within a gaseous mixture in concentrations ranging as high as 30 mole percent or greater. Removal of the carbon dioxide by scrubbing with alkaline solutions, e.g., aqueous amine solutions, is usually prohibitive when the concentration of the carbon dioxide exceeds about two or three mole percent.
The separation of components of different boiling points by distillation usually provides advantages but the separation of carbon dioxide from liquefied hydrocarbon streams is quite burdensome because carbon dioxide crystallizes, solidifies or "ices up" over a wide range of temperature and pressure conditions, which ranges often overlap or correspond to those required for most effective separation. The formation of a solid phase in a distillation column for obvious reasons is generally viewed as intolerable.
An acute disadvantage in prior art processes employing only a single distillation column for the separation of carbon dioxide from gaseous hydrocarbon streams, notably methane streams, is that distillations conducted at economically feasible conditions leaves significant amounts of the carbon dioxide present and consequently cannot be used when it becomes necessary to remove greater amounts of the carbon dioxide. For example, a cryogenic separation process utilizing a single distillation column described by J. K. Jones in Proceedings of the 43rd Annual Convention, Mar. 25-27, 1974 at Denver, Colo., Pages 171-176, suggests that the removal of carbon dioxide from methane-containing streams to provide methane which contains about 10 mole percent CO.sub.2 is possible by operation at 730-750 psia at temperatures no lower than about -100.degree. F., but that further reduction of the CO.sub.2 content below the 10 mole percent level would require operation close to the solid CO.sub.2 -vapor region, and close to the critical pressure loci (Page 176).
A process for effecting the separation of carbon dioxide from a predominantly methane stream is described in U.S. Pat. No. 2,888,807 to Phillip E. Bocquet. The separation requires the use of two distillation columns, each operated under different sets of conditions dependent on the concentration of carbon dioxide; (a) as ranging below 8 mole percent, or (b) as ranging above 8 mole percent. In each instance the first and second distillation columns, respectively, of the two different types of operation are maintained under the same operating conditions, the respective operations differing only in that the feed is introduced at different locations. Where the carbon dioxide is present in the lower concentrations, the feed is directly introduced into the first column of the series, and where the carbon dioxide is present in the higher concentrations, the feed is directly introduced into the second column of the series.
In each type of operation characterizing the Bocquet process, the first columns are operated at or below the critical temperature of methane such that feeds to a respective column provide a carbon dioxide concentration below that which, on cooling at the operating pressure of the column, would produce a solid carbon dioxide phase. Effluents from the top of the second columns contain substantially the same concentration of carbon dioxide as the feeds to said first columns. The operating pressure applied to said second columns is maintained above a critical pressure defined as that at which the carbon dioxide phase will exist, and above which pressure a solid carbon dioxide phase will not coexist with a vapor. Whereas this process has provided certain advantages over previous processes, it nonetheless possesses acute disadvantages. A notable disadvantage is that two operating columns are required to effect the separation of carbon dioxide from a predominantly methane stream. Moreover, the operation becomes particularly complex when it is required to treat methane streams of varying carbon dioxide concentration ranging above and below 8 mole percent carbon dioxide.